Scintillation analysis system

ABSTRACT

A system for the combustion preparation of samples for scintillation analysis comprising a combustion tube containing a combustion catalyst, a chimney tube positioned within the combustion tube and resting on the catalyst and means for admitting a sample for combustion, oxygen gas for ignition and an outlet means for the combustion product. Also, means for collecting the combustion products comprising: for H2O an externally cooled condenser like system containing means for contacting a scintillation solvent with the gaseous combustion products in a cooled zone; and for CO2 an externally cooled condenser like system containing a helical rotatable band within the inner bore thereof for distributing a scintillation solvent along the walls thereof for countercurrent contact with the CO2 combustion product.

Umted States Patent 1 1 [111 3,776,695

Peterson Dec. 4, 1973 SCINTILLATION ANALYSIS SYSTEM Fisher 1963 Catalog,Modern Laboratory Applican- [75] Inventor: John 1. Peterson, FallsChurch, Va. (1963) Assignee: The of America 88 ri y mi e Moms 0represented by the Secretary Assistant Exanumr-R. M. Reese Department ofHealth, Education Anome), nolman & Stern and Welfare, Washington, D.C.

[22] Filed: Sept. 30, 1970 ABSTRACT 21 App]. No.1 76,705

OTHER PUBLICATIONS Tamers, M. A. et al., Inter. J. Appl. Radiationlsotopes, Vol. 15, pp. 697-702, (1964).

Herberg, R. J., Analytical Chemistry, Vol. 32, pp. 42-46, (1960).

A system for the combustion preparation of samples for scintillationanalysis comprising a combustion tube containing a combustion catalyst,a chimney tube positioned within the combustion tube and resting on the22 Claims, 3 Drawing Figures PMENIEUMB 41ers 3.776.695

SHEEI 1 [IF 2 INVENTOR.

Jo/m Peterson PATENTEI] DEC 4 I975 3.776.695 sum 20F 2 Fig. 3

INVENTOR. John Peterson am c A TTORNE YS 1 SCINTILLATION ANALYSIS SYSTEMBACKGROUND OF THE INVENTION The present invention relates to acombustion system for the scintillation analysis of tritium andcarbon-l4.

Heretofore, various methods have been proposed for preparing samples forscintillation counting. One of the most commonly employed method is theso-called oxygen flask procedure," originally conceived by W.Z. Hempel(Z. Angen. Chem., p. 393, (l892)). This method has been refined over theyears and was most recently simplified by JD. Davidson et al. (Advancesin Tracer Methodology" (S. Rothchild Ed.), Vol. 4, p. 67, Plenum Press,N.Y., 1968).

Briefly, the oxygen flask procedure" involves the combustion of a samplecontaining tritium or carbon- 14 in an atmosphere of oxygen in a glassflask. The products of combustion, namely, carbon dioxide and water, arequantitatively collected employing a suitable solvent and scintillationanalyzed for tritium and carbon-l4. Oxygen combustion of samples,particularly biological materials, is highly desirable for liquidscintillation analysis because of the relative simplicity as comparedwith other methods.

There are, however, drawbacks associated with the oxygen flaskprocedure." These will be apparent from the following detaileddescription of a typical oxygen flask procedure" for the scintillationanalysis of a tissue sample. The primary object of the procedure is tosuspend the dried sample material in the center of a confined quantityof oxygen and to effect its ignition and containment in the oxygenatmosphere with a minimal loss of heat until combustion is complete. Thecarbon dioxide and water produced thereby must be quantitativelycollected in a measured volume of liquid scintillation countingsolution.

Generally, a large heavy walled glass Erlenmeyer filter flask isemployed as the combustion vessel. The dried sample is suspended in aplatinum or similar nonreactive metal mesh basket" held in place byinsertion in a glass rod which is held in place by a rubber stopper inthe neck of the flask. Prior to stoppering, the interior of the flask isflushed for several seconds with oxygen to provide the necessaryatmosphere for combustion. The sample and holder are then positioned inplace by stoppering the flask and the side arm of the flask is closed byslipping a silicone rubber tube thereover and clamping the free end. Thesample is then ignited, preferably employing an infrared light beam.Combustion is generally complete in less than one minute. The flask isthen set aside and allowed to cool for several minutes. Thereafter, thecounting solvent is added to the flask. This is generally accomplishedby attaching a volumetricpipette containing the measured amount ofsolvent to the silicone rubber tube on the sidearm of the flask. Sincethe pressure in the interior of the flask is always negative relative toatmospheric pressure following combustion, the solvent will be drawninto the flask upon release of the clamping means, on the tube. Theflask is then swirled to distribute the solvent over the bottom andlower walls thereof and allowed to cool at about -15 C for up to aboutminutes to permit completion of condensation. Additional solvent isdelivered through the sidearm to rinse any activity which may havesequestered in the solvent that wetted the sidearm initially. The flaskis again swirled to mix the solvent. A measured amount of the solutionis then withdrawn for scintillation counting.

Although the aforedescribed method constitutes a great improvement overother prior art methods for the preparation of samples for scintillationanalysis, there are several disadvantages associated therewith.

First, the gas volume-pressure characteristics of the combustion flaskdictate the size of the sample. For a large Erlenmeyer flask (e.g. 2liters) the upper limit on sample weight is about I00 mg. of combustiblematerial. (Davidson, lbid.) Accordingly, the oxygen flask procedurewould be inapplicable for the analysis of a material requiring a largesample by reason of its low tritium or carbon-l4 content.

Secondly, although most of the pieces of apparatus employed in the"oxygen flask procedure are inexpensive, it is nonetheless necessary toemploy a platinum or similar expensive non-reactive metal sample holderfor positioning the sample in the oxygen atmosphere.

Thirdly, the various time-consuming manipulative steps required in theprocedure, including the necessity for cleaning the system followingeach combustion, preclude the use of this method for analyzing a largenumber of samples in a short period of time.

Thus, it has been reported that a skilled worker can run only about 24analyses per day. It is frequently necessary, however, to performhundreds of analyses in a single series of tests. Employing the oxygenflask procedure" for such analyses would require several 'days.

There exists, therefore, the need in the art for a simple, relativelyinexpensive system for the preparation of samples for the scintillationanalysis of tritium and carbon-l4 by combustion which is capable ofhandling large sample sizes in a relatively continuous manner, therebyenabling a large number of analyses in a short period of time.

It has been recently proposed to use various adaptations of the classicmicroanalytical tube combustion train system (Lippman et al, Monatsch.Chem. Vol. 7, p. 9 (1886)) for preparing scintillation analysis samples.Several such systems include those proposed by Peets et al., Anal.Chem., 32, 1465 (i960); Knoche et al., Anal. Biochem. 12, 49, (I965);Tamers et al., Intern. J. Appl. Radiation Isotopes, I5, 697, (1954);Christman et al., Anal. Chem. 27, 1939 (i955); Van der Laarse et al.,Anal. Chim. Acta, 34, 370 (I966), and Griffith et al., Anal. Biochem,22, 465(l968). None of these adaptations, however, were sufficientlyrapid and convenient and capable of handling large samples so as toconstitute a significant advance over the oxygen flask procedure."

It is an object of the present invention to provide a method whichenables the rapid and efficient oxygen combustion of a great number oflarge samples for scintillation analysis in a short period of time.

it is a further object of the present invention to provide an improvedtube combustion train which is especially adapted for igniting largescintillation analysis samples on a semi-continuous basis.

it is still a further object of the present invention to provide animproved carbon dioxide collection apparatus which is especially adaptedfor collecting the carbon dioxide produced by combustion in the improvedtube combustion train for scintillation analysis.

SUMMARY OF THE INVENTION The method of the present invention comprisespositioning a sample of the material to be scintillation analyzed on aporous substrate, preferably containing a suitable oxidation catalyst,positioned in an elongated, vertical combustion tube which is at a pointapproximately midway between the open ends of the tube. This portion ofthe tube is maintained at a temperature sufficient to promote ignitionof the sample when it is placed in position on the substrate. A verticalchimney tube preferably enlarged at the bottom and open at both ends, ispositioned on the substrate over the sample coaxial with the combustiontube. A stream of oxygen is introduced near the top of the combustiontube and caused to flow through substantially the entire length of thecombustion tube, the chimney tube and the porous substrate. The sampleis ignited and, following complete combustion, the combustion productsare swept by the oxygen stream through the combustion tube and out thebottom thereof. The chimney tube is designed to prevent upward movementof the burned material. the combustion products are then subjected tocooling, preferably in an externally cooled condenser type apparatus andcontacted with a suitable scintillation solvent to effect condensationand dissolution of the active products to be analyzed. The solution isthen subjected to scintillation analysis according to conventionalmethods.

The combustion apparatus of the present invention comprises anelongated, vertical combustion tube having a porous substrate,preferably containing a suitable oxidation catalyst positioned at asubstantially intermediate point in the tube and a vertical chimney tubeopen at both ends and preferably enlarged at the bottom disposed withinand coaxial with the combustion tube and resting on the poroussubstrate. The combustion tube is additionally provided with an oxygengas inlet means near the top thereof, means for closing the top of thecombustion tube and an outlet means near the bottom of the combustiontube.

The apparatus of the present invention also includes means forcontacting the combustion products with a scintillation solventdepending upon the type of scintillation analysis contemplated.

When the sample is to be subjected to tritium analysis, the apparatus isprovided with means for both condensing the active water component ofthe combustion product and effecting its dissolution in a suitable watersolvent. Briefly, such means comprises a tubular jacket surrounding andcoaxial with the outlet portion of the combustion tube and additionallycontaining a solvent inlet means whereby the solvent introduced therebycontacts the combustion products. The tubular jacket and outlet portionof the combustion tube are also in communication with an externallycooled conduit means, e.g., a condenser, wherein further condensationand dissolution of the combustion products in the scintillation solventtakes place.

Where the sample is to be subjected to active CO, analysis, theapparatus is provided with means for delivering the combustion productsto the bottom portion of a carbon dioxide collector apparatus comprisingan externally cooled vertical tube, e.g. condenser. Positioned withinthe inner bore of the vertical tube and adapted for rotation thereinabout its longitudinal axis is an elongated strip of inert materialwhich has been provided with a permanent twist about its longitudinalaxis to form a planar surface in the shape of a helix. The tube isclosed at its upper end and provided with a suitable carbon dioxidesolvent inlet means and oxygen outlet means at the top thereof and aliquid outlet means at the bottom thereof for delivering of a solutioncontaining the active material to be analyzed. The combustion productsand solvent are contacted countercurrently in the condenser like tube,the contact being enhanced by the rotation of the helix shaped elementwhich forms a thin film of the solvent along the inner walls of thetube. The active carbon dioxide containing solution is delivered throughthe outlet means at the bottom of the condenser for scintillationanalysis according to conventional methods.

DETAILED DESCRIPTION OF THE INVENTION The advantages associated with themethod and apparatus of the present invention will become apparent fromthe following description and claims and from the accompanying drawingswherein:

FIG. 1 is a front elevational view of one embodiment of the combustionand water collector apparatus.

FIG. 2 is a front elevational view of a more specific embodiment of thecombustion and water collector apparatus.

FIG. 3 is a front elevational view of the active carbon dioxidecollector apparatus.

Referring to the drawings, FIG. 1 illustrates a typical combustion andactive water collector apparatus according to the present invention. Theapparatus comprises a vertical elongated combustion tube 1 preferablycomposed of heat resistant glass having an opening 2 at the top forintroduction of a sample of the material to be analyzed. A stopperingmeans, not shown, is also provided for closing the opening 2 duringcombustion to prevent contamination of the combustion products. Thecombustion tube 1 is provided at a point near the top with an oxygen gasinlet 3 in the form of a side arm tube which communicates with theinterior of the combustion tube. The combustion tube 1 is provided witha suitable porous substrate 5 at a point approximately midway betweenthe inlet opening 2 and the outlet opening 7. The substrate 5 preferablycontains a porous oxidation catalyst layer below the surface upon whichthe sample to be ignited is positioned. Disposed within combustion tube1 and coaxial therewith is a vertical chimney tube 4 which is open atboth ends and rests upon porous substrate 5. The chimney tube 4 ispreferably enlarged at the bottom and funnel shaped at the top whichterminates at a point below gas inlet means 3. The diameter of thechimney tube 4 below the funnel shaped top portion is less than that ofthe inner bore of the combustion tube 1. The outer periphery of thefunnel shaped top of the chimney tube may, if desired, be adapted for aclose fitting relationship with the inner periphery of the inner bore ofcombustion tube 1. Surrounding the middle portion of the combustion tube1 including the porous substrate 5 and the lower portion of the chimneytube 4 is a heating means 6, e.g. a furnace, which does not comprise apart of the present invention. At the lower end of the combustion tube 1there is provided an outlet means 7, preferably in the form of theconstricted end portion of the combustion tube 1.

The active water collector apparatus comprises a vertical tube orconduit means 8 coaxial with and in close fitting relationship at thetop end thereof with the lower portion of combustion tube 1 so as toform a chamber 9 into which the outlet means 7 extends. Tube 8 isprovided with a scintillation solvent inlet means in the form of a sidearm tube which communicates with chamber 9 in said tube 8. Chamber 9 isexternally cooled by a cooling element 11 and terminates in an outletmeans 12.

P16. 2 represents a preferred embodiment of the combustion and watercollector apparatus of the present invention. The apparatus comprises anelongated. vertical combustion tube in two sections 1' and 7. Thecombustion section 7' and the chimney 11' to be described in more detailhereinafter are formed of quartz. All other glass portions of theapparatus are formed of heat resistant, preferably borosilicate, glass.The upper portion 2' of the upper section 1' of the combustion tube isfunnel shaped to provide a sample inlet means. The tube section 1' isprovided at a point just below the sample inlet means with a stopcockassembly 3. The frictionally engaged surfaces of the stopcock assembly3' are preferably ground and of sufficient high quality that lubricationis unnecessary, thereby avoiding contamination of the sample.

The stopcock means 3' is provided with an innerbore 4' whichcommunicates with the inner bore of the tube section 1'. The inner bore4' is closed at one end by means of sealing element 5' to provide asample holding chamber within the bore 4' of the stopcock.

Just below stopcock 3', tube section 1 is provided with an oxygen gasinlet means 6 in the form of a side arm tube which communicates with theinterior of tube section 1'. Oxygen gas may be supplied to thecombustion tube through inlet means 6 in regulated amounts by means ofapressure regulator, needle valve and flow meter combination, not shown.

The upper section 1' of the combustion tube apparatus is joined to thelower or combustion section 7 by means of a ground glass joint 8',preferably nonlubricated. An air tight sealing of sections 1' and 7' maybe enhanced by providing a tensioned spring means 9' which is attachedto both sections 1 and 7 by means of hooks 10'.

Disposed within and coaxial with combustion section 7' is chimney tubell. Chimney 11' is open at both ends and rests on a porous bed l2 ofinert chips, preferably quartz, upon which the sample material ispositioned for combustion. Chimney II is preferably enlarged at thelower portion thereof and funnel shaped at the top. Preferably, thefunnel shaped top of the chimney ll communicates with the bottom ofupper combustion tube section 1' so that most of the oxygen gas streamfrom inlet means 6' passes through chimney 11'. The diameter oftheremainder of the chimney 11, including the enlarged lower portionthereof is less than that of the inner bore of tube section 7'. Thechimney 11' is essential to the method and apparatus of the invention inthat it causes the combustion of the sample to be analyzed to occurslowly and smoothly and prevents pyrolyzate from passing into andcondensing in the upper section 1' of the apparatus, thereby fouling thesystem.

Positioned below bed I2 is a porous preliminary catalyst layer l3,preferably composed of copper oxide wire. Disposed below layer 13' isprimary oxidation catalyst layer 14, preferably composed of a mixedoxide of copper and manganese (hopcalite, Lamb et al., J.lnd.Eng.Chem.,l2, 2 l3 (l92())). The layers l2, l3 and 14' are separated by means ofporous elements 15'. preferably composed of a heat resistant, inertmaterial, e.g. quartz wool.

It has been found that the rapid and complete combustion of a sample inthe oxygen stream cannot be obtained without the use of a catalyst. Thethermal pyrolysis of a sample in the absence of a catalyst producesunburned volatile material which would be carried through the system tocondense on cooler portions of the apparatus. Nor is this phenomenoncompletely avoided by employing extremely high temperatures. Inaddition. the burning of a sample in a short period of time produces somuch gaseous combustion product that the oxygen atmosphere in thevicinity of the sample becomes highly diluted thereby resulting inincomplete combustion. By employing a catalyst such as a mixed oxide ofcopper and manganese, the necessary oxygen for complete combustion ofthe sample is provided by the catalyst which is then regenerated laterby the oxygen stream.

The upper porous layer 12' functions as a combustion platform. Thepreliminary catalyst layer 13 serves to indefinitely increase the lifeof the primary catalyst layer 14'. The top surface of layer 13' becomessintered from the heat of combustion following repeated ignitions andshould be cleaned and/or replaced at regular intervals particularlywhere pyrolyzate condensation occurs in the chimney 11'. With regularinspection and frequent cleaning of layers 12 and 13', primary catalystlayer 14' should last indefinitely.

Surrounding the major portion of lower combustion tube 7' is a heatingelement 16 such as a tube furnace (e.g. Lindberg model 55035-A,Hevi-Duty Heating Equipment Company), which supplies the heat necessaryfor combustion. It is to be understood that any heating means capable ofigniting the sample and maintaining combustion may be employed.Preferably, the heating means should be capable of providing atemperature of about 600 C in the region of the combustion tubesurrounded by the heater.

The bottom of lower tube section 7' terminates in a constricted ortapered end portion or outlet means 17'. The furnace or heating element16 should be positioned such that the outlet means 17' is keptsuffciently warm to evaporate water of condensation which condensesthereon during sample burning. Obviously, the furnace should also bepositioned so as to prevent condensation of any type within lower tubesection 7' or chimney 1].

Attached in tight-fitting relationship with said lower tube section 7'is'the water collector apparatus of the invention. The apparatuscomprises a vertical tube 18' preferably composed of heat resistant,borosilicate glass, coaxial with lower combustion tube 7'. The tube 18'is positioned so as to surround constricted outlet means 17 forming anopen chamber extending around the outer periphery thereof. The tube 18is joined to lower tube section 7' above outlet means 17 by means of aground glass joint which is preferably lubricated with silicone grease.The joint is strengthened by means of tensioned spring member 19' whichis, attached to lower tube section 7' and tube 18' at each end by meansof hooks 20'. Tube 18 is provided with a scintillation solvent inletmeans 2| in the form of a side arm tube which communicates with theinterior of tube 18' above outlet means 17'. The outlet tip 17' ispositioned to fit closely into the opening of collector spiral tube 23'which is disposed within vertically disposed cooling means or condenser24'. The upper opening of condenser 24' is positioned so as tocommunicate with the lower openingof tube 18' at 22'. Outlet tip 17'should be so positioned as to form a liquid trap around the end portionthereof, when solvent is introduced therein. This prevents combustiongases from entering the space between the joint 22' and the outletmeans. The collector spiral tube 23' is preferably composed of glass orquartz and is coiled within condenser 24' so as to allow for maximumcooling when coolant (e.g. alcohol) is introduced into the condenser viainlet means 25 and flowed around the coil and out through outlet means26'. Preferably, the temperature of the coolant is maintained at about Cby a refrigerating unit, not shown. The collector spiral tube terminatesin a drain tube 27' which preferably has an internal flared portion 28'which allows the solvent to rinse the inner surfaces thereof completelywithout ejecting with violent splashing. A hemispherical cup member 29'is positioned in tight fitting relationship around said drain tube 27'to prevent atmospheric water condensate on the outer surface of thecondenser 24' from dripping into the container employed to catch thescintillation solution sample.

Scintillation solvent passes through the collector apparatus by means ofinlet 21' in regulated amounts by employing conventional liquid meteringmeans, not shown. Suitable scintillation solvents for water includemixures of methanol and toluene and mixtures of dioxane and toluene. Itis to be understood, however, that any scintillation solvent possessinga sufficient degree of water acceptance may be employed.

The above described system may be operated as follows:

A sample of the material to be analyzed is placed into inner bore 4 ofstopcock 3' through inlet means 2'. The oxygen flow is begun I liter perminute at psig. The scintillation solvent is continuously pumped intothe collector apparatus through inlet means 21' at a rate of ml over a 2minute period and the stopcock 3 is rotated so as to drop the sampleonto layer 12' within chimney ll. The primary catalyst layer 14' ispreferably a 50 mm deep bed of mixed oxide of copper and manganese whichhas been ground and sieved to 16/30 mesh and maintained in thecombustion tube at about 600 C 50 C by heating element 16'. The samplebursts into flame within 10 to l5 seconds, although some pyrolysis ofthe sample occurs with the production of combustion products prior tothe appearance of a flame. Burning continues smoothly over approximatelythe first minute of operation. The procedure is allowed to continue fortwo minutes to ensure complete combustion of the sample and thoroughrinsing of the collector coil. The tritiated water of combustion passesthrough outlet means 17' in vapor form, condenses and dissolves in thescintillation solvent in the collector spiral tube 23' and passesthrough outlet means 27' into a suitable container for scintillationanalysis.

The chimney tube is essential to the operation of the apparatus. In itsabsence, the sample burns irregularly and partially burned materialscoat the upper walls of the combustion tube. Moreover, upward movementof the combustion gasses is prevented by the nature of the flow of theoxygen down through the chimney tube.

The porous layer 12 is preferably composed of quartz which serves toshield the catalyst layer from the high temperatures of burning. Thepreliminary catalyst 13' is preferably copper oxide whereas the primarycatalyst layer 14 is preferably copper and manganese.

The advantages of the method and apparatus of the invention arenumerous.

First. since the combustion tube apparatus is openended, i.e.,non-pressurized, as is the case with the above-described "oxygen flaskprocedure" there is virtually no limit on the size of the sample whichmay be ignited. It will be recalled from the above discussion that thesample size in the oxygen flask procedure is dictated by the gasvolume-pressure characteristics of the flask. Since the present systemis non-pressurized, there is no limit on the sample size other than thatwhich may be practically introduced into the system. Whereas the oxygenflask procedure" is limited to the ignition of samples whose weight isno more than about 300 mg. of combustible material, the sample of theinvention has been employed to ignite samples having a weight of over500 mg. of combustible material. Accordingly, the method and apparatusof the present invention may be employed in the scintillation analysisof sample materials containing small amounts of tritium which requirethe utilization of large sample sizes.

Secondly, the apparatus of the present invention may be constructed ofrelatively inexpensive and readily available materials.

Thirdly, and probably most importantly, the method and apparatus of thepresent invention permit an operator to conduct a large number of sampleignitions and collections in a short period of time as compared with theoxygen flask procedure." Whereas the latter requires numerous timeconsuming manipulative steps including the necessity for cleaning thesystem following each combustion, the system of the present inventionmay be operated on a semi-continuous basis. Thus, following a firstcombustion and collection of sample, the system is immediately ready fora second combustion. All of the combustion products have been sweptthrough the system and collected in the solvent stream and removedthrough the drain tube. Since there is no necessity for opening thesystem to contamination from the surrounding atmosphere and sinceperfect conditions for combustion and solvent collection are maintainedby the constant oxygen and scintillation solvent flow, the system isadapted for the combustion of a new sample approximately every threeminutes. Whereas the oxygen flask procedure" is adapted to permit thecombustion of only about 24 samples per day, the system of the presentinvention is adapted for the combustion of more than samples per day.

it will be further recalled from the above discussion that severalattempts have been made to modify the classic microanalytical tubecombustion train system for preparing scintillation analysis samples.The present system is not to be confused with these previousmodifications, however. None of the systems reported to date are capableof effecting the combustion of a great number of samples having a largesize in such a short period of time.

The samples are conveniently introduced into the combustion apparatus incapsule form, i.e., enclosed in plastic or gelatin capsules. The samplesmay be utilized in any desired form. For example, the samples may bepowdered, liquid, slurry-like, dried, etc. The system of the inventionis most readily adapted for the analysis of biological samples such astissues. etc. it is to be understood, however, that any material capableof combustion to produce active water and carbon dioxide may be preparedfor scintillation analysis in the system and method of the invention.

Although the use of both gelatin and plastic capsules of various typeshave been employed, it has been found that capsules composed ofpolycarbonates are superior to any other tested. The polycarbonatescombine the most valuable properties of light weight, strength, goodcombustion characteristics and a low contribution of scintillationquenchers. They have been found to be superior to the often used gelatincapsules which must be hardened by treatment with formaldehyde prior touse, are relatively heavy and which produce nitrogen oxides uponcombustion which have a quenching effect.

Biological tissues are preferably emulsified or homogenized in water andpreferably dried prior to combustion. This is most convenientlyaccomplished by introducing a predetermined amount of the homogenizateinto the larger end of the capsule. Since the system of the invention isadapted for the combustion and collection of more than a hundred samplesper day, all of the samples to be analyzed in a given period of time arepreferably prepared in advance. The capsule ends are then placed on ahot plate, preferably in shallow depressions to avoid spillage, and thewater content thereof evaporated by heating at about 8090 C.

The invention will be further illustrated by the following non-limitingexamples.

In each of the following examples a furnace temperature of 600 C 50 C,an oxygen flow of L0 liters per min. and a solvent flow of 20 ml in 2min. were employed.

EXAMPLE I A liquid scintillation spectrometer (No. 4322 PackardInstruments Co., lnc.) was employed in the following tests: The windowsettings used were: 40-l00 at 60 percent gain for tritium counting andl,000 to infinity at 6 percent gain for the automatic externalstandardization count (AE5). These settings provided a closelyproportional relation between sample counting efficiency and AES.

The internal standard method was employed to standardize the system forwater recovery and quench correction. A standard sample similar inmaterial and identical in size to the samples being analyzed andcontaining an exactly known activity, is burned and counted. The ratioof tritium counts per minute (cpm) divided by AES is determined for eachstandard and each sample. The activity of each sample in disintegrationsper minute (dpm) is given by the following relationship:

cpm w (rs simple (maimed X AES cpm )standard Table 1 shows the typicalscintillation counting efficiencies observed with a solvent comprising amixture of dioxane and toluene (Herberg, J. Anal. Chem., 32, 42 (l960)).

TABLE 1 Typical Counting Efficiencies Using Diotol Solvent Sample Rangeof counting efficicncy 1" PLO in unoxygenatcd solvent 20-22 PLO inoxygen sat. solvent (no sample combustion) 15-18 Filter pulp in Lexancapsule (no nitrogen oxides) l5-l6 Wool felt in gelatin capsule(nitrogen oxides from protein) l0-l 3 R=recover correction y AESutnblarned an ard x (ABS cpm burned standard Q= quench correction= (dpm)unbumed standard AES 1 X (cpm unburned x (AES sample standard The aboverelationships given for quench correction are based upon a proportionalrelation between the cpm and AES for a sample. If any other method ofdetermining quench correction is used, it may also be combined with therecovery correction as shown.

EXAMPLE 2 This example is designed to illustrate the lack of a carryoverof a small fraction of a sample to contaminate the following sample in acontinuous combustion operation where a series of samples are to beanalyzed. Such a carryover effect would cause serious errors where a lowactivity sample is burned after a high activity sample. The results ofthe test are set forth in Table 2 In runs Nos. 1-20 various sized piecesof filter tablet were placed in gelatin capsules and 50 pl of tritiatedtoluene were then added. An empty capsule containing no activity wasburned between each sample containing tritium. Diotol solvent and amixed copper-manganese oxide catalyst were used in all tests in thisexample. The hopcalite contained eight atoms of manganese to one ofcopper, although some variation in this ratio is acceptable. in thisseries of tests, the carryover of activity from a tritiated sample to afollowing inactive sample did not exceed 0.] percent. Diotol has thefollowing composition: toluene (350ml), methanol (210 ml), dioxane (350ml), naphthalene (73 g), PPO (4.6 g), POPOP (0.08 g).

Runs No. 21-36 were similar to 1-20, exception being the use oftritiated biotin in ethanol solution to represent a nonvolatile, slowburning form of tritium, and the substitution of discs of wool felt forthe filter tablet. The wool felt was employed to provide a closersimulation to the protein content of tissue samples. The 32-: carryoverin the second series did not exceed 0.2 per- 6 2 cent. There being noevidence of a cumulative effect in 7 I014. HRH these tests, samples maybe run consecutively and cong tinuously without intermediate washingsteps. 5 |u1.7 100.4

7' TABLE II Test 0! Analytical Behavior 0! the Method Activity, AverageSam le c.p.m. Counting collection weig t, backgrd. Carryover, emclency,Recovery efliciency,

Number Sample mg. i0 c.m.p.= percent percent percent percent 6 IPreceded by eight samples 0! II-toluene ranging from to 470 mg. Includescapsule weight 0! 150 v Counts were 1 mln., giving a coe cient ofvariation of i.0%.

EXAMPLE "3 This Example compares the results obtained with the system ofthe invention with those obtained by the above-described oxygen flaskprocedure." The samples were rat liver homogenate from animals closedwith tritiated biotin suspended in 0.25 M sucrose solution. For thedeterminations according to the invention, L00 ml samples of homogenatewere evaporated at 90 C in hardened gelatin capsules. The dried samplesweighed about 200 mg. including the capsule without cap. The oxygenflask combustions were done on 1.00 ml. samples of homogenate dried incellulose bags under infrared lamps. Methanol-toluene solvent was usedin both procedures. The determinations according to the invention werecarried out in duplicate, and compared with average duplicatedeterminations of each sample by the oxygen flask method. A singlecounting efficiency correction factor was used for all the oxygen flaskdeterminations. Each determination according to the invention wascorrected individually for a counting efficiency which was between 8 and10 percent. The tritium activity of the samples averaged 150,000 dpm,and they were counted for 1 minute with a coefficient of variation 0.3percent. The results are set forth in Table 3.

TABLE lll SAMPLE k recovery based on oxygen flask values for duplicatedct. by combustion tubc method l IOLO. 99,3 '2 100.8, 100.8 3 103.3.100.3

4 Since these are internal standards, average recovery is B Determinedby comparison with standards in solvent.

These results demonstrate the superiority of the system of the presentinvention to the oxygen flask method in that similar results areobtained although the system of the invention is much simpler to operateand is capable of handling a great number of samples in a short periodof time.

It should be further pointed out that the danger of explosion is greatlyreduced in the system of the present invention, inasmuch as the oxygenmixes with the solvent only in the cold collector portion of the system,where the vapors are carried out the drain tube.

The present invention also includes an improved carbon dioxide collectorsystem for collecting active carhon-l4 for scintillation analysis. Thesystem may be used in conjunction with the above-described combustiontube system wherein samples are burned yielding active carbon dioxide.

in the oxygen flask procedure," the carbon dioxide is absorbed over aperiod of a half hour or more, using Woellers phenethylamine solvent.(Woeller, Anal. Biochem. 2, 408, (l962)). Other methods includeabsorption of the radioactive carbon dioxide in cold scintillationsolvent by diffusion or bubble trap systems or by freezing out the CO,in a liquid nitrogen trap with subsequent solvent absorption. Thesemethods are effective but slow and inefficient and do not enable thecollection of a large number of samples in a short period of time. Thesystem according to the invention permits the collection of carbondioxide efficiently from the relatively large volume of oxygen flow fromthe above-described combustion system in a relatively small volume ofscintillation solvent.

Referring to FIG. 3, the condenser like assembly is mounted verticallybeside the combustion tube. A delivery tube 1" preferably of Teflon, isconnected to the tapered outlet 17' of the combustion tube of FIG. 2with a short piece of tubing (not shown), preferably of silicone rubber.The delivery tube 1" is sealed into the bottom of the collector assemblyand its tip pointed upward therein. The assembly is provided with asolvent inlet means 9" and an oxygen outlet means at the top thereof.The solvent pump means (not shown) and rate of solvent flow is the sameas described for water collection. The delivery tube 1 is positionedwithin tube 3" which is completely enclosed by condenser-like tube 4"which is provided with coolant inlet and outlet means 5" and 6" Disposedwithin tube 3" is helical band 7" The helical band 7" is adapted forrotation within tube 3" around spinner shaft 8" Shaft 8" extends throughand is adapted for free rotation within stopper ll The helical band 7"is .shaped such that the edges thereof contact the inner walls of thetube 3" in such a manner that, upon rotation, the band maintains a thin,smooth film of solvent on the walls afier introduction through inlet 9"The solvent film must be such that no bare zones or pools of solventform on the walls.

Generally, smaller bores of tubing, lower rotational speeds of thehelical band and higher temperatures of operation tend to decrease theefficiency of the system.

By employing a sufficiently large bore in tube 3" which provides asufficiently slow flow velocity of oxy! gen, moderate variations inrotational speed of the helical band 7" and cooling temperatures, aswell as imperfections in the sweep of the glass wall by the edges of theband, are not critical. Generally, the diameter of the inner bore oftube 3" may vary from about to about mm, a diameter of about 17.4 mmbeing preferred. The rotational speed of the helical band 7" may varyfrom about 2,500 to about 4,000 rpm with a speed of tzq tsgsw r be heldat about 0 C.

The bottom of the tube 3" is provided with delivery means 12" and liquidtrap 13''. The liquid trap 13" functions in the same manner as the cupmember of 29' of FIG. 2 to prevent condensate from the atmosphere fromdripping into the container employed to catch the sample.

The helical band 7" is preferably constructed by tearing strips of 100mesh stainless steel screen to the proper width and setting a twisttherein by mounting in a lathe and carefully rotating. A helixconsisting of two strips may be assembled onto a length of stainlesssteel rod (8" by spot welding the ends together with reinforcing strips.The edges of the screen are then feathered by removing a few strands ofwire and the desired fit to the glass wall is achieved by gentle bendingof the feathered edge.

The operation of the system for carbon dioxide collection is identicalto that described for water collection with the exception that adifferent solvent is used and the solvent flow precedes the dropping ofthe sample into the combustion tube. In the case of tritiated watercollection, the solvent flow is started at the same time a sample isdropped into the combustion tube. in the CO, collector, the sample isnot dropped into the combustion tube until the flow of solvent hasreached the bottom of the collector assembly.

The scintillation solvent used comprised toluene (430 ml) methanol (300ml), phenethylamine (270 ml), 2.5-diphenyloxazole (PPO) (5 g), dimethyl[2,2 phenylene-bis-(5-phenyloxazole)] (POPOP) (0.5 g). The collectionsystem absorbs nitrogen oxides very sirciently, which causes a seriousquenching of the scintillation solvent unless dimethyl POPOP isemployed.

As with water collection, the sample size capacity of the system isdetermined by the capacity of the solvent to absorb C0,. If the samplesize is larger than about 400 mg including the capsule, precipitationtends to occur in the collector. There is no decrease in the efficiencyof the absorption of the C0, when precipitation occurs; however, if theamount of precipitate is large it will not be completely washed out ofthe collector in one pass. In such cases, a second or third solventrinse cycle may be necessary to remove all of the precipitate. Theformation of precipitate is not detrimental as long as it is followed bysufficient clear solvent to carry all of the precipitate to countingcollector. Generally, a sample size up to about 400 mg. is suitable forCO, collection.

The system may be employed for the collection of tritiated water and/orcarbon dioxide. It is not particularly desirable to employ the CO,collection system for the recovery of water, however, unless it isdesired to count both isotopes together in the same solvent, because thecounting efficiency for tritium is about half of that obtained when thewater collector is used.

Quench and recovery corrections are made as described above inconnection with water collection. in the following examples the windowsettings in the counter used for CO, counting with the abovedescribedsolvent were: 40 to L000 at 20 percent gain for the carbon l4 count andL000 to infinity at 50 percent gain for the automatic externalstandardization count.

AMBLM containing identical doses of carbon-l4 were con ducted. Thesamples were evaporated in cellulose bags for the oxygen flask procedureand in hardened gelatin capsules for the combustion method of theinvention. The carbon- 1 4 was added in the form ofmethylglyoxalbis-guanyhydrazone-C which had demonstrated a failure tolose activity under the evaporation conditions employed. In thecombustion method of the invention, a mixed copper-manganese oxidecatalyst was employed. The results are set forth in Table 4.

The two methods agree within 1 percent; their precision being identical.Inasmuch as the CO, collection system of the present invention iscapable of handling a large number of samples in a relatively shortperiod of time as compared with the oxygen-flask method, the inventionrepresents a significant improvement over the prior art.

In general there are several key features which make this combustionsystem different than prior art systems and which are essential to itssuccess. These features include the use of hopcalite, the use of achimney in the combustion chamber to control the flow of oxygen andcombustion gasses, the encapsulation of the sample in TABLE IV Oxygenflask method Rapid combustion method Corrected. Percent SampleCorrected. Percent Percent Sample and size c.p.m. recovery site I c.p.m.recovery carryover Plasma, 1.5 ml 27, 622 99. 8 l 7 ml 27, 941 100.9 0.19 27, an 98. 2 28, 014 101. 2 16 27, 505 99. 28, 128 101. 6 l6 Caress,100 mg 27, 863 100. 6 121 mg 27, 785 1013 2- Small intestine, 347 mg 27,320 98. 7 231 mg 27,364 08.8 34 27, 015 97. 6 27, 792 100. 4 21 27, 36098. 8

Large intestine, 138 mg 27,578 28, 171 101 7 28 27, 692 a 0 28, 03 0 27,951 100. 9 34 Kidney, 50 mg 26, 996 27, 729 100. 0 92 27, 282 27, 44699. 1 64 27, 570 27, 766 100. 3 45 Liver, 150 mg 27,510 27, 770 11113 9027, 056 27, 98. 7 90 27131 27, 775 100. 3 41 Muscle, 96 mg 27,345 98. 863 mg. 27,538 99.5 27,711 00.1 27,746 1012 .16 27, 2B6 98. 5

Mean 27, 408 90. D 27, 690 100. 0 34 Samples were of mouse tissuehomogenized in distilled water. dry weights of tissue given (exeoptlorthe plasma, which was human, volume before drying given). 7

Counts per minute (c.p.m.) values were corrected by subtractingbackground (19 c.p.m.) and by reducing each to a common countingelfielency oi 65%. The rapid combustion method values were corrected fora collection efliciency 0191.493. The oxygen flask values were correctedfor the aliquot factor and the effect on counting ellielency of adlflercnt volume in the counting vial than with the rapid combustionsamples. The c.p.m. were based on 10 min. counts.

1 For comlparlson purposes. the mean c.p.m. oi the rapid combustionmethod is assumed to represent 100.0%

recovery, e separately measured absolute recovery 0 a This re resentsthe percent of activity of that sample whic burned an collectedimmediately following the aetlvc sample.

convenient containers which also serve as evaporation vessels, and theunique design of collectors to mix the water and carbon dioxide withscintillation solvent. All other combustion systems which have beendeveloped for scintillation analysis have been impractical because ofthe lack of simple, rapid, direct means of automatically absorbing thecombustion products in scintillation solvent.

What is claimed is:

l. A combustion method for the preparation of a test sample forscintillation analysis comprising providing a vertical elongated tubehaving a porous substrate positioned at a substantially intermediateportion within said tube and a vertical chimney tube open at both endsdisposed within and coaxial with said vertical tube and resting on saidporous substrate, said porous substrate including a catalyst whichcomprises a mixed oxide of copper and manganese, positioning saidintermediate portion within a heating zone, maintaining said heatingzone at 600 C: 50 C, positioning a sample of the material to be analyzedwithin said chimney tube on said porous substrate, introducing a streamof oxygen into said vertical tube at a point near the top of said tubeand flowing said stream of oxygen through substantially the entirelength of said vertical tube and said chimney tube, igniting said sampleand recovering a combustion product from a point below said poroussubstrate.

2. A method according to claim I wherein said porous substrate comprisesan upper layer comprising an inert porous material, an intermediatelayer comprising a porous copper oxide preliminary catalyst and a lowerlayer comprising a porous mixed oxide of copper and manganese.

3. A method according to claim 1, comprising introducing a stream ofsolvent for at least one of the products of combustion of said sampleinto said vertical 97.47 is corrected for in b.

it appears in an untagged sample (empty capsule) tube at a point belowsaid porous substrate, and causing said oxygen and solvent streamscarrying said products of combustion to flow from said vertical tube ata point near the bottom of said tube into an externally cooled zone toeffect the condensation of said products of combustion and thedissolution of at least one of the products in said solvent, therebyproducing a sample suitable for scintillation analysis.

4. A method according to claim 3, wherein said products of combustioncomprise carbon dioxide and water.

5. A method according to claim 3 wherein said solvent comprises amixture of methanol and toluene.

6. A method according to claim 3 wherein said solvent comprises amixture of dioxane and toluene.

7. A method according to claim 3 wherein said vertical chimney tube isenlarged at the bottom.

8. A method according to claim 3 wherein said material comprises anorganic material.

9. A method according to claim 8 wherein said organic material comprisesa biological material.

10. A combustion apparatus suitable for the preparation of samples forscintillation analysis comprising a vertical elongated combustion tubehaving a porous substrate positioned at a substantially intermediatepoint within said combustion tube and a vertical chimney tube open atboth ends disposed within and coaxial with said vertical combustion tubeand resting on said porous substrate; said vertical combustion tubehaving a gas inlet means at a point near the top of said verticalcombustion tube and an outlet means below said porous substrate; saidporous substrate including a catalyst which comprises a mixed oxideofcopper and manganese.

11. The apparatus of claim 10 wherein said vertical combustion tubecontains a solvent inlet means at a point intermediate said poroussubstrate and said outlet means.

12. The apparatus of claim 10 further comprising external cooling meansfor said outlet means.

13. The apparatus of claim 10 wherein said vertical combustion tube isequipped with a stopcock means at a point above said gas inlet, saidstopcock means containing an inner bore closed at one end whichcommunicates with the inner bore of said vertical combustion tube.

14. The apparatus of claim 10 wherein said chimney tube is enlarged atthe bottom.

15. The apparatus of claim 10 wherein said chimney tube is funnel shapedat the top.

16. The apparatus of claim 10 wherein said porous substrate comprises anupper layer comprising an inert porous material, an intermediate layercomprising a porous copper oxide preliminary catalyst and a lower layercomprising a porous mixed oxide of copper and manganese.

17. The apparatus of claim 16 wherein said layers are separated byporous inert separators.

18. The apparatus of claim 17 wherein said inert sep- 18 aratorscomprise quartz wool.

19. The apparatus of claim 10 wherein said outlet means comprises aconstricted end portion of said vertical combustion tube.

20. The apparatus of claim 19 wherein said solvent inlet means comprisesa vertical tube coaxial with and surrounding said constricted endportion of said vertical combustion tube, said vertical tubecommunicating around the periphery thereof, with said combustion tube ata point above said constricted end portion and having a side arm inletconduit between said joint and said constricted end portion.

21. The apparatus of claim 19 wherein said outlet means is incommunication with an externally cooled conduit means for condensing anddissolving products of combustion formed in said vertical combustiontube.

22. The apparatus of claim 21 wherein said conduit means is in the shapeof a vertical, spirally coiled tube coaxial with said verticalcombustion tube and terminates in an internally flared outlet means.

$ i i i I

2. A method according to claim 1 wherein said porous substrate comprisesan upper layer comprising an inert porous material, an intermediatelayer comprising a porous copper oxide preliminary catalyst and a lowerlayer comprising a porous mixed oxide of copper and manganese.
 3. Amethod according to claim 1, comprising introducing a stream of solventfor at least one of the products of combustion of said sample into saidvertical tube at a point below said porous substrate, and causing saidoxygen and solvent streams carrying said products of combustion to flowfrom said vertical tube at a point near the bottom of said tube into anexternally cooled zone to effect the condensation of said products ofcombustion and the dissolution of at least one of the products in saidsolvent, thereby producing a sample suitable for scintillation analysis.4. A method according to claim 3, wherein said products of combustioncomprise carbon dioxide and water.
 5. A method according to claim 3wherein said solvent comprises a mixture of methanol and toluene.
 6. Amethod according to claim 3 wherein said solvent comprises a mixture ofdioxane and toluene.
 7. A method according to claim 3 wherein saidvertical chimney tube is enlarged at the bottom.
 8. A method accordingto claim 3 wherein said material comprises an organic material.
 9. Amethod according to claim 8 wherein said organic material comprises abiological material.
 10. A combustion apparatus suitable for thepreparation of samples for scintillation analysis comprising a verticalelongated combustion tube having a porous substrate positioned at asubstantially intermediate point within said combustion tube and avertical chimney tube open at both ends disposed within and coaxial withsaid vertical combustion tube and resting on said porous substrate; saidvertical combustion tube having a gas inlet means at a point near thetop of said vertical combustion tube and an outlet means below saidporous substrate; said porous substrate including a catalyst whichcomprises a mixed oxide of copper and manganese.
 11. The apparatus ofclaim 10 wherein said vertical combustion tube contains a solvent inletmeans at a point intermediate said porous substrate and said outletmeans.
 12. The apparatus of claim 10 further comprising external coolingmeans for said outlet means.
 13. The apparatus of claim 10 wherein saidvertical combustion tube is equipped with a stopcock means at a pointabove said gas inlet, said stopcock means containing an inner boreclosed at one end which communicates with the inner bore of saidvertical combustion tube.
 14. The apparatus of claim 10 wherein saidchimney tube is enlarged at the bottom.
 15. The apparatus of claim 10wherein said chimney tube is funnel shaped at the top.
 16. The apparatusof claim 10 wherein said porous substrate comprises an upper layercomprising an inert porous material, an intermediate layer comprising aporous copper oxide preliminary catalyst and a lower layer comprising aporous mixed oxide of copper and manganese.
 17. The apparatus of claim16 wherein said layers are separated by porous inert separators.
 18. TheapparaTus of claim 17 wherein said inert separators comprise quartzwool.
 19. The apparatus of claim 10 wherein said outlet means comprisesa constricted end portion of said vertical combustion tube.
 20. Theapparatus of claim 19 wherein said solvent inlet means comprises avertical tube coaxial with and surrounding said constricted end portionof said vertical combustion tube, said vertical tube communicatingaround the periphery thereof, with said combustion tube at a point abovesaid constricted end portion and having a side arm inlet conduit betweensaid joint and said constricted end portion.
 21. The apparatus of claim19 wherein said outlet means is in communication with an externallycooled conduit means for condensing and dissolving products ofcombustion formed in said vertical combustion tube.
 22. The apparatus ofclaim 21 wherein said conduit means is in the shape of a vertical,spirally coiled tube coaxial with said vertical combustion tube andterminates in an internally flared outlet means.